For convenience of description the present invention will be described with reference to the POCSAG (Post Office Code Standardisation Advisory Group) or CCIR No. 1 code format for use in digital radiopaging systems. However, the present invention is equally applicable to any similar data transmission system wherein message information is transmitted in code word format preceded by an addressing code word for destination identification, for example the Electronic Engineering Association (EEA) preferred data transmission system code.
The capabilities of complex digital codes, such as the POCSAG code, include the ability to perform some degree of error detection and, more importantly, some degree of error correction. The exact extent of these properties is a function of the particular code format in question. The POCSAG code, for instance, has a capability of detecting 11 bits in error if in a burst or 5 randomly distributed bits in error in each 32 bit code word and correcting up to two errors if randomly distributed within the 32 bit code word or up to 5 bit errors if grouped as a burst in any one code word.
The advantages in terms of improvement in code word decoding success rate introduced by the capability to correct bit errors in coded data are significant. However, such capabilities have drawbacks in that it becomes possible to incorrectly modify erroneous data to valid data if such erroneous data has a large number of errors, that is, more errors than the code is capable of correcting. This is an undesirable disadvantage.
For the POCSAG code, it is only possible to correct one bit error in each 32 bit code word and remain within the specification for falsely correcting code words. This limitation is undesirable.